This invention, in one of its aspects, relates to microwave radiation absorbing materials, and particularly to electromagnetic wave absorbers known as Jaumann absorbers.
In another of its aspects the invention relates to means for fabricating Jaumann absorbers.
Technological advances in radar systems, in anti-missile systems and in surface-to-air anti-aircraft pose severe penetration problems for missiles, bombers, fighters and surveillance aircraft. To counteract such measures microwave absorbers must be used. Microwave absorbers reduce the radar cross-sections of such airborne objects by attenuating reflectivity values.
Electromagnetic wave absorbers of the general type contemplated herein are described in such patents as U.S. Pat. Nos. 2,875,435 and 2,822,539. Specifically, the Jaumann absorber is the subject of British T.R.E. Report No. T 1905. In these absorbers laminated layers are disposed on the face of the reflecting surface, the laminated layers being lossy layers separated by dielectric spacing layers.
Cross-section reduction is accomplished through arrangement of the layers so that electromagnetic wave energy incident on the outer surface falls first upon an outer dielectric layer, the function of which is to create an extra null in the reflection level. The energy next proceeds through a partially conductive lossy layer and then through another dielectric layer. As many reflection reducing nulls exist as there are dielectric layers. The existence of the extra null makes the peaks lower.
As set forth in U.S. Pat. No. 2,822,539 this type of microwave radiation absorber is in the form of flat sheeting designed so as to provide a progressive decrease of the electrical index of refraction from the back surface to the front surface. It is stressed in that patent that difficulties such as the accuracy necessary for the construction of this type of absorber have limited its use. As another example of these difficulties, in the British T.R.E. Report describing the Jaumann absorber it is pointed out that the conductivity of the lossy layers was very difficult to control. In addition a special glue was necessary for sticking the layers together. Because of the critical nature of such absorber fabrication it has not been possible to produce an absorber of this type having a reflection loss greater than 30 decibels, particularly a broad banded absorber with such a reflection loss.